Question: Will Novak Djokovic and Andy Murray join Rafael Nadal and Richard Gasquet in the men's semifinals at the U.S. Open? The answer is coming today.

Murray is up first, with an afternoon match against No. 9 seed Stanislas Wawrinka. As defending champion, Murray figures to have support at Arthur Ashe Stadium. However, Wawrinka should test Murray in the players' 14th meeting.

Djokovic, first seed in the men's draw, faces No. 21 seed Mikhail Youzhny of Russia in the late match on the main show court.

Doubles teams also are in the spotlight. Bob and Mike Bryan are under way against Leander Paes and Radek Stepanek. It's a strong challenge for the American brothers and No. 1 seeds.

Serena Williams and sister Venus also play this afternoon, and theirs is a stern test as well. Sara Errani and Roberta Vinci of Italy are the top seeds in women's doubles.

Murray knows he is in for a test from Wawrinka, having gone 8-5 against him. Wawrinka has surged in recent months and beat Djokovic in the Australian Open.

"I don't know if he's working harder," Murray told the BBC. "He made a coaching change around the clay-court season, which may have helped him because he was without a coach for quite a while. That could have helped him a bit.

"The match he had with Novak in Australia at the beginning of the year, that could have contributed to it as well. There are a lot of things that go into it.

"But he hasn't changed any of his strokes technically or anything. You'd expect most of it to be confidence."

Women's semifinals will be played Friday, with Serena Williams facing China's Li Na.